1. Field of the Invention
The present invention relates to an automatic tool changer to be mounted in a machine tool and, more particularly, to a tool changer having a tool magazine which is driven by a servo motor.
2. Description of Related Art
Complex machine tools such as machining centers which are capable of performing a plurality of machining operations such as milling and drilling operations are generally mounted with an automatic tool changer which is adapted to select a proper tool from a plurality of types of tools and automatically attach and detach the tool to a rotary tool spindle.
Such an automatic tool changer typically includes a tool magazine having a plurality of tool pots arranged in a closed loop, and is adapted to rotatively drive the tool magazine in accordance with an indexing command outputted from a programmable controller (PMC) on the basis of a machining program for positioning a predetermined tool retained in one of the tool pots in a tool changing position, and then exchange a tool attached to the rotary tool spindle and the tool positioned in the tool changing position with the use of a tool changer arm provided between the tool changing position and the rotary tool spindle in accordance with a tool changing command outputted from the PMC on the basis of the machining program.
The conventional automatic tool changer employs a general-purpose motor as a drive source for rotatively driving the tool magazine and, therefore, a mechanical lock mechanism is additionally provided for arresting the tool magazine in a proper angular position after the rotation of the tool magazine. In recent years, a servo motor having a retention capability has been employed instead of the general-purpose motor. The retention capability of the servo motor makes it possible to arrest the tool magazine in the angular position after the rotation of the tool magazine. This is advantageous in that the aforesaid mechanical lock mechanism can be obviated.
Where the tool magazine is to be arrested in the angular position by the retention capability of the servo motor, however, power supply to the servo motor should be continued during the arrest of the tool magazine, thereby entailing a greater power consumption than in the case where the general-purpose motor is employed for the rotation of the tool magazine.
Where the servo motor is employed as the driving source for the tool magazine, an electromagnetic brake which is operable when the power supply to the servo motor is stopped is generally provided for preventing unintended rotation of the tool magazine at emergency stop of the servo motor. During the arrest of the tool magazine which requires the power supply to the servo motor, power consumption is entailed for release of the electromagnetic brake. Therefore, the total power consumption is further increased.
The tool magazine and the servo motor are generally coupled with each other via ordinary gears (e.g., spur gears) rather than via a ball screw, so that the servo motor is significantly influenced by a torque of the tool magazine. The tool magazine typically has a rotation orbit defined in a vertical plane. Where a relatively heavy tool is held in an upper tool pot when the tool magazine is arrested, the weight of the tool produces a torque in the tool magazine. The torque is suppressed by the retention capability of the servo motor, so that the servo motor is subjected to an unbalanced load for a long period of time. Accordingly, there is a possibility that the servo motor is overheated during the arrest of the tool magazine, through there is no problem during the rotative driving of the tool magazine.
Therefore, a servo motor having a greater steady torque should be employed as the drive source for the tool magazine in consideration of the unbalanced load generated during the arrest of the tool magazine.
It is therefore an object of the present invention to provide an automatic tool changer which employs a servo motor as a drive source for a tool magazine and, yet, features a minimized power consumption and a reduced steady torque of the servo motor.
In accordance with a first aspect of the present invention to achieve the aforesaid object, there is provided an automatic tool changer which comprises a tool magazine to be rotatively driven by a servo motor and is adapted to perform a tool magazine indexing operation in accordance with an indexing command and then perform a tool changing operation in accordance with a tool changing command, wherein power supply to the servo motor is temporarily stopped if the next indexing command is not outputted within a predetermined time period after completion of the tool changing operation and, when the next indexing command is outputted, the power supply to the servo motor is resumed.
In the automatic tool changer having the aforesaid construction, the power supply to the servo motor for driving the tool magazine is kept stopped until the next indexing command is outputted after a lapse of the predetermined time period from the completion of the tool changing operation. Therefore, a power consumption required for the servo motor to arrest the tool magazine and a power consumption required for release of an electromagnetic brake can be reduced as compared with the conventional automatic tool changer in which the power supply to the servo motor is continued during the arrest of the tool magazine.
In accordance with a second aspect of the present invention, the power supply to the servo motor is temporarily stopped if the next tool changing command is not outputted within the predetermined time period after completion of the tool magazine indexing operation and, when the next tool changing command is outputted, the power supply to the servo motor is resumed. This arrangement reduces the power consumption required for the servo motor to arrest the tool magazine and the power consumption required for the release of the electromagnetic brake before the next tool changing command is outputted after a lapse of the predetermined time period from the completion of the indexing operation. Therefore, the power consumption of the automatic tool changer can further effectively be reduced.
Further, the electromagnetic brake is operative to prevent the tool magazine from being significantly offset from an index position during the stop of the power supply to the servo motor. When the power supply to the servo motor is resumed upon the output of the tool changing command, a positional offset of the tool magazine, if any, can immediately be corrected. The tool changing operation, even if achieved by direct tool exchange between the tool magazine and a rotary tool spindle, can smoothly be performed with out significantly influencing a tact time.
As described above, the tool magazine is retained not by the retention capability of the servo motor but by the electromagnetic brake during the arrest of the tool magazine, i.e., until the next indexing command is outputted after the lapse of the predetermined time period from the completion of the tool changing operation and until the next tool changing command is outputted after the lapse of the predetermined time period from the completion of the indexing operation. Even if a relatively heavy tool is located in an upper position of the tool magazine, the servo motor is not subjected to an unbalanced load. Therefore, a servo motor having a smaller steady torque can be employed as the servo motor for driving the tool magazine without consideration of the overheat of the servo motor which may otherwise occur during the arrest of the tool magazine.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.